MUSE is an implementation of a "multiple mounts merging" filesystem.
It's something like unionfs, but it allows writing to all the underlying filesystems at the same time.
It is suitable for convenient unification of large disk arrays when fault-tolerance is not the top priority.

badsector implements the algorithm described in http://smartmontools.sourceforge.net/BadBlockHowTo.txt for the ext[2-4] partitions.
The smartmontools can report problematic bad sectors in LBA addressing format. This program relocates bad LBA sectors, provided they are unused.

Robinhood Policy Engine is a multi-purpose tool for managing the content of large filesystems. It can audit filesystem content, perform accounting, remove old unused files according to admin-defined policies, show customizable alerts based on file properties, backup data to external storage, and more. It has advanced capabilities for Lustre filesystems. It leverages OST usage, and lists or purges files per OST, with policy criteria based on pools and OST index. It can also process MDT changelogs with Lustre v2. Originally developped for HPC, it has been designed to perform all of its tasks in parallel, so it is particularly adapted for running on large filesystems with millions of entries and petabytes of data. But you can nonetheless take advantage of all of its features for managing smaller filesystems.

posixovl is a FUSE filesystem that provides POSIX functionality (UNIX-style permissions, ownership, special files) for filesystems that do not have such, for example vfat. It is a modern equivalent of the UMSDOS filesystem.

Parrot and Chirp are user-level tools that make it easy to rapidly deploy wide area filesystems. Parrot is the client component: it transparently attaches to unmodified applications, and redirects their system calls to various remote servers. A variety of controls can be applied to modify the namespace and resources available to the application. Chirp is the server component: it allows an ordinary user to easily export and share storage across the wide area with a single command. A rich access control system allows users to mix and match multiple authentication types. Parrot and Chirp are most useful in the context of large scale distributed systems such as clusters, clouds, and grids where one may have limited permissions to install software.

Cmdfs is a FUSE virtual filesystem that applies an arbitrary filter command to selected files in a source directory tree to create the destination files.
It includes configurable caching and monitoring of the source tree to limit CPU use and prefetch data.